The following definitions are herewith defined:                3GPP—Third Generation Partnership Project        HAT—Hybrid Access Terminal for CDMA1x and EV-DO        VoIP—Voice over Internet Protocol        CS—Circuit Switched        IMS—Integrated Multimedia Service        SIP—Session Initiation Protocol        S-CSCF—Serving Call Signalling Control Function        MGCF—Media Gateway Control function        MGW—Media Gateway        RTP Real-time Transport Protocol        HOF—Handoff Function/SIP Proxy        PDSN—Packet Data Support Node        GGSN—Gateway GPRS Support Node        
With the advance of broadband wireless communication technologies, more and more broadband wireless access networks are emerging. CDMA1x EV-DO, IEEE 802.11 based WLAN, and IEEE 802.16 based WiMAX, are just a few examples of recent technology advancements. The use of Voice over Internet Protocol (VoIP) over these broadband wireless access networks is considered cost efficient and enables rich call services. However, conventional circuit switched voice networks, such as CDMA1x or GSM, are likely to serve as the primary voice services in the near future. As a result, hybrid solutions that are capable of leveraging emerging broadband wireless access technologies and which are also backward compatible with the more conventional circuit switched networks are desirable.
At present, CDMA-1x EV-DO (hereinafter “EV-DO”) has gained industry support. Typically, EV-DO is deployed as an overlay network with a CDMA-1x network, thus providing an overlaid coverage area. One reason for the overlay deployment of EV-DO is to provide a mechanism for offloading traffic from the CDMA-1x networks, especially data services related traffic. In order to enable network access in an overlaid coverage area, the use of hybrid access terminals (HATs) that are capable of both EV-DO and CDMA-1x network access is preferred. EV-DO Rev. A enhances the quality of service (QoS) support, particularly for VoIP. As a result, VoIP over EV-DO (VoIP/EV-DO) Rev. A is being considered as a desirable feature for service providers or network operators.
When two HATs, for example HAT-A and HAT-B, are engaging in IMS calls, the media path is peer-to-peer. In other words, the real-time transport protocol (RTP) end points are at HAT-A and HAT-B. To enforce QoS treatment for the forward direction media traffic, HATs send packet filters to the corresponding PDSNs. The packet filter typically comprises a RTP source IP address and a port number. When an RTP packet from HAT-B reaches the PDSN-A for HAT-A, the packet filter is used to match the RTP packet to the ongoing packet flow between HAT-B and HAT-A and header compression is applied. The compressed RTP packet is then sent to a flow exhibiting an acceptable delay sensitive QoS for receipt by HAT-A.
There exist several models for adoption into the 3GPP and 3GPP2 standards to deal with handover from a VOIP call to a switched circuit voice call. According to the dynamic anchoring model, in order to prepare one of the HATs, for example HAT-B, to hand down to a circuit switched voice call, a media gateway (MGW) is put in the middle of the RTP path as the anchor point. From the perspective of HAT-A, this means that the RTP end point is changed during the call, giving rise to the following issues.
First, it can be noted that the original packet filter in PDSN-A is no longer valid because the RTP packets arriving at HAT-A are now coming from the media gateway. As a result, there is a need to update the packet filter on PDSN-A. Secondly, there is an interval between the moment when the MGW is interposed into the RTP path and the time when the media gateway starts to send RTP packet to HAT-A. During this interval, the original packet filter in the PDSN-A is still valid but needs to be updated to reflect the existence of the MGW. During this interval, it is probable that one or more packets will be misdirected resulting in an unacceptable loss of data.
Such a problem is not unique to communications wherein a MGW is interposed between two HATs. The same problem exists in other situations, such as when the remote data end point is changed during a data connection session. For example, a SIP user agent may change devices during an IMS session, such as in the instance of a call transfer. In order to provide seamless service continuity in such events, the packet filter in the PDSN (3GPP2) or GGSN(3GPP) should be updated.